dsh-screenshot-diff: A DSH plugin for pixel diffing existing screenshots

# Introduction When engaging in interface development or having agents operate on pages, a question often arises: Has the page actually changed after this modification? Visually comparing two screenshots is prone to missing details, and writing a pixelmatch script from scratch every time is too repetitive. dsh-screenshot-diff is a workflow plugin within the DeepSeek Harness (DSH) ecosystem. The philosophy of DSH is "everything is a plugin," and repetitive verification actions are perfect for being encapsulated as plugins: a single command takes two existing screenshots as input and outputs a diff image with red-marked differences, a labeled tri-comparison image, and a JSON snippet that can be consumed by scripts. It does not handle screenshot capture; the input must be two screenshots you have already obtained. # What is dsh-screenshot-